1. Field of the Invention
The invention relates in general to elevator systems, and more specifically to hydraulic elevator systems which utilize load weighing devices.
2. Description of the Prior Art
The jack assembly and power units of a hydraulic elevator system are selected according to the size and capacity of the cab desired by the user, the rise, and the desired speed. The cab and its sling, collectively called an elevator car, has a predetermined weight, and a predetermined rated capacity, and the plunger of the jack assembly initially selected to accommodate the gross weight and rise has a predetermined weight per foot. Thus, the gross weight is calculated and checked with jack capacity to ensure that the jack selected is proper for the application. If not, the next higher jack size is used in the calculations, until the proper jack size is found. Using the gross weight, area of jack bottom and seal friction pressure, the pressure required at the jack is calculated. The pressure loss from the power unit to the jack is then calculated, using the distance from the power unit to the jack, the elbows, etc., to determine the pressure required at the power unit in order to provide the required pressure at the jack assembly. The power unit capable of providing this pressure is then selected, which thus selects the horsepower and frame of the electric drive motor, and the pump size.
The calculations include a factor of safety which permits occasional overloading of the elevator car without damage to the equipment. However, if the environment in which the elevator system is to be installed is such that it may be subjected to frequent and/or excessive overloads, beyond about 25% of rated load, it would be desirable to monitor car load and to prevent the operation of the elevator car when the load exceeds a predetermined value. Also, when the load in the elevator car reaches rated load, it would improve service if hall calls are by-passed. Prior art arrangements for measuring the load in an elevator car, however, are quite costly, usually detecting the deflection of a yieldable platform via a plurality of switches adjusted to be actuated in response to a predetermined deflection, and thus load, on the platform. The switches are difficult to adjust, because of their location, and a required periodic readjustment to ensure accuracy.